IMPACT-AFFECTED QUARTZ CLASTS IN THE PROXIMAL EJECTA DEPOSIT, WETUMPKA IMPACT STRUCTURE, ALABAMA

A slump-emplaced, proximal ejecta deposit (boulder-bearing polymict breccia) occupies an area of a few thousand square meters on the crater floor of the Late Cretaceous marine-target impact structure at Wetumpka, Elmore County, Alabama. Relatively rare quartz clasts, ranging from pebbles to boulders, occur in the sandy clay matrix of this ejecta deposit. Many of these quartz clasts are impact-affected, and these quartz clasts derive from three possible sources in the target: (1) quartz augen and other quartz masses within the basement Piedmont schists; (2) a coarse residuum layer atop these schists where quartz augen and masses were concentrated prior to deposition of directly overlying Upper Cretaceous target sediments (Tuscaloosa Group); and (2) fluvial pebbles from the Tuscaloosa Group. Generally, clasts from (1) are angular; (2) are subangular; and (3) are subrounded to rounded. All impact-affected quartz clasts bear notable petrographic evidences of shock, including PFs, incipient PDFs, FFs, and/or toasting. A minor proportion of the subrounded, rounded, and subangular quartz clasts display an impact-induced textural zonation, wherein there is an outer zone dominated by intersecting PFs that appears white and a darker inner core that has a glassy luster and is dominated by PFs and related incipient PDFs and FFs. Megascopic evidence of impact effects include (1) lower (‘pumice-like’) density; (2) various autobreccia textures; (3) remarkable contorted clast shapes; and/or (4) 60-degree cleavage in large quartz crystals. Impact-affected quartz pebbles, cobbles, and boulders have been reported from a few other impact structures, but the range of such impact-related features in quartz clasts at Wetumpka appears to be unusual.